One of the known functions of serum albumin is the binding and transport of various molecules in the blood circulation, including fatty acids, bilirubin, hormones, metal ions and other endogenous and exogenous compounds with widely differing properties (Murayama and Tomida, 2004). Among other substances, various drugs are known to bind to one of two main high affinity sites in albumin (Kragh-Hansen et al., 2002). It is also a very soluble protein, stable at pH ranging from 4 to 9 and at temperatures over 60° C., and available in high purity at a reasonable cost (Peters, 1977, 1985). These properties, and especially the natural affinity of a great number of therapeutic drugs to the protein, suggest that albumin may be used as a versatile carrier protein for various applications in the clinic. Some of the drug delivery technologies that make use of albumin as a drug carrier were recently reviewed by Kratz (2008).
There are several examples for the use of albumin in drug delivery systems, for example after incorporation of the albumin and the drug in a polymeric delivery system (Murray et al., 1983).
The covalent conjugation of Poly-(ethylene glycol) (PEG) to albumin, named PEGylation, was first described in 1977 (Abuchowski et al.) for purposes of reducing the immunogenicity of the protein. Other bioactive molecules, including various drugs, have also been PEGylated in order to prolong their residence time in the blood stream by improving their physical and thermal stability, increasing their protection against proteolysis and decreasing their clearance (Caliceti and Veronese, 2003; Greenwald et al., 2003).
PEGylated albumin hydrogels for drug delivery applications were previously described in the literature (Gayet and Fortier, 1995, 1996; Wacker et al., 2006; Iza et al., 1998). However, in these cases the release rate from the hydrogels was dominated by Fickian diffusion and not controlled by the specific affinity of the various drugs examined to albumin.
Hydrogels may be chemically designed to allow sustained release in response to environmental changes, such as temperature induced phase transition (Wang et al., 1999). Another way to control release kinetics of a drug from a hydrogel is by designing its swelling properties, based mainly on the synthetic polymer properties, including volume fraction, molecular weight between cross-linking points and the corresponding mesh size (Peppas et al., 2000). The degradation rate of the network may also be designed, based on the bioactive component, inducing controlled release of drug molecules (Huynh et al., 2008).
Tissue engineering, i.e., the generation of new living tissues in vitro, is widely used to replace diseased, traumatized or other unhealthy tissues. The classic tissue engineering approach utilizes living cells and a basic scaffold for cell culture. Thus, the scaffold structure attempts to mimic the natural structure of the tissue it is replacing and to provide a temporary functional support for the cells.
Tissue engineering scaffolds are fabricated from either biological materials or synthetic materials, such as polymers. Synthetic materials such as polyethylene glycol (PEG), hydroxyapatite/polycaprolactone (HA/PLC), polyglycolic acid (PGA), poly-L-lactic acid (PLLA), polymethyl methacrylate (PMMA), polyhydroxyalkanoate (PHA), poly-4-hydroxybutyrate (P4HB), polypropylene fumarate (PPF), polyethylene glycol-dimethacrylate (PEG-DMA), beta-tricalcium phosphate (beta-TCP) and polytetrafluoroethylene (PTFE) provide precise control over the mechanical properties of the material (Drury and Mooney, 2003).
Scaffolds made of PEG are highly biocompatible and exhibit versatile physical characteristics based on their weight percent, molecular chain length, and cross-linking density. In addition, PEG hydrogels are capable of a controlled liquid-to-solid transition (gelation) in the presence of cell suspension. Moreover, the PEG gelation (i.e., PEGylation) reaction can be carried out under non-toxic conditions in the presence of a photoinitiator or by mixing a two-part reactive solution of functionalized PEG and cross-linking constituents (Lutolf and Hubbell, 2003).
WO 1995/015352 describes a hydrogel comprising albumin and bifunctionalized polyethylene oxide. WO 2005/061018 describes a scaffold comprising a naturally occurring protein such as fibrinogen cross-linked by PEG, by attaching modified PEG molecules to cysteine residues of the protein. WO 2008/126092 describes a scaffold comprising albumin attached to at least two synthetic polymers.
A single PEG molecule conjugated to an albumin protein molecule through an amino group, as a model for assessing the abilities of a quantitative assay of PEG, is described by Nag et al, 1996.